Thermal treatment for the stress-relief of titanium alloy parts

Abstract

The invention relates to a process for the preparation of a part made of titanium alloy, comprising a thermal treatment for relaxing the internal stresses of the part, the thermal treatment comprising maintaining at a temperature “T 1 ” greater than the beta transus (beta transition) temperature, referred to as “Tbt”, and the part being free to deform by creeping. The invention also relates to a tool for carrying out this process.

1 . A process for the preparation of a titanium alloy part, wherein the process comprises a thermal treatment for relaxing the internal stresses of a titanium alloy part which has been subjected beforehand to at least one thermomechanical transformation stage, the process being characterized in that the thermal treatment comprises maintaining at a temperature “T 1 ” greater than the beta transus (beta transition) temperature, referred to as “Tbt”, and in that the part is free to deform by creeping. 
     
     
         2 . The process as claimed in  claim 1 , wherein titanium alloy is of alpha-beta type. 
     
     
         3 . The process as claimed in  claim 2 , wherein maintaining at the temperature T 1  is carried out for a time sufficient to make possible the complete transformation in the alloy of a close-packed hexagonal microstructure to a body-centered cubic microstructure. 
     
     
         4 . The process as claimed in  claim 3 , wherein the temperature T 1  is greater by at least 5° C. than Tbt. 
     
     
         5 . The process as claimed in  claim 1 , wherein it comprises maintaining at the temperature T 1  for a period of time of 5 to 120 minutes. 
     
     
         6 . The process as claimed in  claim 1  wherein, subsequent to maintaining at the temperature T 1 , cooling is carried out with a cooling rate of greater than 5° C./min. 
     
     
         7 . The process as claimed in  claim 1 , wherein the part made of titanium alloy is positioned for the stress-relaxation treatment in a shaping tool comprising one or more pattern cavities calibrated to receive a part to be relaxed. 
     
     
         8 . The process as claimed in  claim 7 , wherein the shaping tool is made of at least one single or composite material, the thermal inertia of which is greater than that of titanium or of the titanium alloy and the variations in size of which related to creeping at the temperature T 1  are virtually nonexistent. 
     
     
         9 . The process as claimed in  claim 8 , wherein the shaping tool is made of concrete or composite concrete. 
     
     
         10 . The process as claimed in  claim 1 , wherein it comprises, prior to the stress-relaxation thermal treatment stage, at least one rough machining stage. 
     
     
         11 . The process as claimed in  claim 10 , wherein the rough machining stage or stages are carried out in order to treat virtually all, of the surfaces of the parts. 
     
     
         12 . The process as claimed in  claims 10 , wherein, during the rough machining stage, the part is flattened and shaped against at least one reference support. 
     
     
         13 . The process as claimed in  claim 10 , the part is flattened and shaped against said reference support by flattening, against the reference support, one or more flashes formed around the part and resulting from an upstream stamping stage. 
     
     
         14 . The process as claimed in  claim 1 , wherein it comprises, after the stress-relaxation thermal treatment, one or more stages of final machining of the titanium alloy part. 
     
     
         15 . The process as claimed in  claim 1 , wherein the titanium alloy is a TA6V titanium alloy. 
     
     
         16 . The process as claimed in  claim 1 , wherein the part is a slender part. 
     
     
         17 . A shaping tool comprising a shaping region comprising one or more calibrated pattern cavities in order to shape, by creeping, one or more slender parts and/or parts with large differences in cross section of titanium alloy, said shaping tool being composed of at least one single or composite material, the thermal inertia of which is greater than that of titanium or of the titanium alloy and the variations in size of which related to the creeping at a temperature of 1060° C. are virtually nonexistent. 
     
     
         18 . The tool as claimed in  claim 17 , wherein it is composed of concrete and optionally comprises, in addition, curved stainless steel fibers distributed isotropically in the concrete. 
     
     
         19 . The tool as claimed in  claim 17 , wherein it has dimensions so that the cooling rates are substantially constant from one slice, part plus tool, to another. 
     
     
         20 . The tool as claimed in  claim 17 , wherein the pattern cavity region is calibrated in order to shape, by creeping, a slender part exhibiting a slenderness of greater than 10 and/or different cross sections, the variation in cross section of which is greater than 2/1. 
     
     
         21 . The tool as claimed in  claims 17 , wherein the titanium alloy is of the type comprising an alpha-beta phase. 
     
     
         22 . The tool as claimed in  claim 17 , wherein the titanium alloy is a TA6V alloy. 
     
     
         23 . The tool as claimed in of  claims 17 , wherein the pattern cavity comprises at least two supporting surfaces on which the part to be relaxed can at least partially rest. 
     
     
         24 . The tool as claimed in  claim 17 , wherein the pattern cavity comprises a positioning stop formed in each calibrated pattern cavity, the other end of the mold being free, in order to allow the part to freely deform by creeping, or comprises a stop positioned by taking into account the thermal expansion coefficient of the part to be relaxed before and after relaxation thermal treatment, in particular in order to make possible free deformation by creeping. 
     
     
         25 . The process as claimed in  claim 1 , employing the tool as defined in  claim 17 . 
     
     
         26 . The tool according to  claim 23  wherein said supporting surfaces are positioned so that, when a titanium alloy part is maintained at a temperature T 1  greater than its temperature Tbt, said part can be positioned by creeping with greater contact on the supporting surfaces.